Such mechanisms are in particular used for automobile vehicle seats for adjusting the height of these seats. In such an application, the seat is attached to the floor of the vehicle or, more precisely, to the slides attached to the floor, for adjusting the forward-rearward position of the seat, by links hinged on the seat and the said slides respectively. These links are more or less parallel and comprise the two sides of a deformable quadrilateral of a form close to that of a parallelogram. The pivoting of these links therefore especially causes a vertical movement of the seat. Conventionally, only one of these links is comprised of an arm of the type concerned by the invention where the pivoting is controlled, the other link necessarily following the movement of the controlled arm. Seat height adjustment is achieved by rotating the pinion of the mechanism which causes, by meshing with the toothed segment, the pivoting of the hinged arm and therefore a vertical movement of the seat. Conventionally, the drive pinion is installed on the seat, the center of curvature of the circular toothed segment then being located at arm hinge axis on the seat. The operation would however be similar if the center of the toothed segment was located at the hinge axis of the arm on the slide and if the pinion was installed on the said slide.
We know of a seat height adjustment system including such an arm equipped with a toothed segment with external teeth. The pinion meshing with this toothed segment is rotated either manually, or by an electric motor and a reduction gear. On account of the reversibility of the pinion-toothed segment system, the seat is held in the position selected by the user by means ensuring the rotational locking of the said pinion. When control is ensured by motor and reduction gear, this locking can be ensured for example by means of an unreversible reduction gear, for example of the worm screw type. The rotational locking of the pinion can also be ensured by locks. Seat height adjustment systems also exist where the seat is pushed upwards by a spring. In this case, the pinion does not drive the toothed segment but only locks it in position. Adjustment is achieved, after having unlocked the pinion, by pressing the seat against the action on the spring, or by retaining it, until desired position is reached and retention in selected position is ensured by then relocking the pinion.
In all cases, the loads exerted on the seat and retransmitted to the links and the floor connecting arm are supported by the pinion locking means. These loads can be very high if the vehicle is subjected to a shock or involved in an accident. The components ensuring this locking must therefore be oversized, to avoid the rotation of the pinion under the effect of the torque generated by these loads, in comparison with that which would be sufficient in terms of mechanical strength to ensure the normal operation of the mechanism. The result is substantial overall dimensions and weight for these items and high costs.